The use of dichroic dyes in solution with various liquid crystal materials, e.g., nematic liquid crystals and blends thereof, and use of these so-called "guest-host" mixtures in liquid crystal devices, e.g., liquid crystal displays, are well known. In liquid crystal displays, the host liquid crystal material has its orientation controlled by the application of electric fields and in turn interacts with the guest dichroic dye to induce a cooperative conforming alignment. The dichroic dyes manifest low or high absorbance of incident light depending upon their molecular orientation with respect to the light wave vector. Generally, when an electric field is applied to the display, the area between the electrodes appears clear, i.e., all the dyes exhibit minimum absorption, and in the absence of an electric field the display appears dye colored, i.e., the dyes are in a high absorbing state.
Dyes suitable for such guest-host mixtures must have high order parameters and proper solubility in the host liquid crystal material, and the guest-host mixtures should provide good viewing characteristics (e.g., high contrast), high extinction, and chemical and photochemical stability.
Several patents describe dichroic azo dyes that have N,N'-unsubstituted dihydroperimidines as end group moieties. For example, Huffman et al, in U.S. Pat. No. 4,565,424, describe poly(arylazo) dyes containing a 2,3-dihydro-2,2-disubstituted perimidine moiety; Etzbach et al, in U.S. Pat. No. 4,667,020, describe diazo dyes containing a 2,3-dihydro-2,2-disubstituted perimidine moiety; and Cole, Jr. et al, in U.S. Pat. No. 4,122,027, describe a pentaazo dye containing a 2,3-dihydro-2,2-disubstituted perimidine moiety. In each of these cases the nitrogen atoms at the 1 and 3 positions of the dihydroperimidine are bound to a hydrogen atom.
Azo dyes containing dihydroperimidine moieties unsubstituted at the nitrogen positions exhibit relatively low solubility characteristics in nematic hosts. The N--H bonds in the dyes result in relatively high crystalline melting points and thus solubility is reduced. The inventor has studied the solubility of such dyes and discovered that azo dyes having N,N'-disubstituted-2,3-dihydro perimidine end groups, particularly those with alkyl substitution at the 2-position, are more soluble than the same dyes having N--H bonds. Guest-host mixtures prepared with such dyes have excellent properties, most notably improved contrast.
Azo dyes having N,N'-disubstituted-2,3-dihydro perimidine end groups, however, require for their formation 2,3-dihydro-1,3-dialkyl perimidines. Prior to this invention, this class of compounds had not been readily accessible, and thus, the polyazo dyes having end groups derived therefrom had not been prepared.
2,3-dihydro-2,2-dialkyl perimidines cited in the Huffman, Etzbach and Cole patents are readily accessible by condensation of 1,8-naphthalene diamine with a ketone or aldehyde as outlined in FIG. 1. Up to now, however, N,N'-dialkyl-1,8-naphthalene diamines have not been readily accessible.
Two routes are known to prepare 2,3-dihydro-1,3-alkyl perimidines. As a preliminary step, N,N'-dialkyl-1,8-naphthalene diamine is synthesized. One route for synthesizing N,N'-dialkyl-1,8-naphthalene diamines, outlined in FIG. 5 (A. F. Pozharskii et al, Zh. Org. Khim., 16, (10) 2216 (1980) and Chemistry of Heterocyclic Compounds, Plenum Press, New York p 782 (1975) (Translated from Khim. Geterot. Soed., No. 6, 849 (1973))), requires condensation of 1,8-naphthalene diamine with acetic anhydride to give 2-methyl perimidine. Alkylation of the perimidine with dimethyl sulfate and basification with ammonia gives 1,2-dimethyl perimidine; alkylation with methyl iodide gives the methiodide salt; and hydrolysis gives the desired N,N'-dimethyl-1,8-naphthalene diamine. A second route, outlined in FIG. 3, requires the reduction of the diacetamide of 1,8-naphthalene diamine (R. W. Alder, et al, J. Chem. Soc., Pt. 1, 2840 (1981)). 2,3-Dihydro-1,3-dialkyl-2,2-dialkyl perimidines can then be made by condensation of N,N'-dialkyl-1,8-naphthalene diamines with aldehydes or ketones as outlined in FIG. 2. Both methods of forming N,N'-dialkyl-1,8-naphthalene diamines have limitations and have not been found to be applicable to the synthesis of a variety of N,N'-dialkyl-1,8-naphthalene diamines.
The inventor has developed a new route to N,N'-dialkyl-1,8-naphthalene diamines, described below, making it possible to readily, easily and economically prepare such compounds in high yields and to prepare dichroic dyes having end groups derived from such compounds.